The present invention relates to a registration control technique for detecting and correcting color differences for a plurality of images formed in different colors by image forming means, in a multiple image forming apparatus, such as a tandem color copier or printer, which comprises a plurality of image forming means, or an image forming apparatus that sequentially forms a plurality of images having different colors with use of at least one image forming means, and forms a color image by transferring the thus-formed images onto a transfer belt, paper on a transfer belt, or an intermediate transfer material.
Recently, as formation of documents to be handled in the office, or elsewhere, in color becomes popular quickly, there has been increasing usage of image forming apparatuses that produce the documents in color, such as copiers, printers, and facsimiles. The current tendency is for these color image forming apparatuses to be arranged so as to produce high quality color images at high speed, as the quality and processing speed of office paper work increases. Various types of so-called tandem color image forming apparatus have already been proposed or have become commercially introduced as color image forming equipment that meets the previously-described requirements. Specifically, the color image forming equipment is provided with image forming units for respective colors, e.g., black (K), yellow (Y), magenta (M), and cyan (C). Images that are formed in different colors by the image forming units are transferred onto a transfer material to be transported or onto an intermediate transfer material in a superimposed manner, so that a color image is formed.
For example, the following image forming apparatus may be mentioned as the previously-described tandem color image forming apparatus. The tandem color image forming apparatus is provided with four image forming units: namely, a black image forming unit 200K for forming an image in black (K), a yellow image forming unit 200Y for forming an image in yellow (Y), a magenta image forming unit 200M for forming an image in magenta (M), and a cyan image forming apparatus for forming an image in cyan (C), as shown in FIG. 39. These four image forming units 200K, 200Y, 200M, and 200C are horizontally arranged at given intervals. A transfer belt 202 which transports transfer paper 201 to respective transfer positions of the image forming units 200K, 200Y, 200M, and 200C while electrostatically attracting it, is disposed as an endless transfer-material carrier below these four image forming units 200K, 200Y, 200M, and 200C for four colors black, yellow, magenta, and cyan.
The aforementioned four color image forming units 200K, 200Y, 200M, and 200C, i.e., black, yellow, magenta, and cyan, are all constructed in the same manner. These four image forming units 200K, 200Y, 200M, and 200C are arranged to sequentially form a black toner image, a yellow toner image, a magenta toner image, and a cyan toner image in the manner as has been previously described. Each of the image forming units 200K, 200Y, 200M, and 200C is provided with a photosensitive drum 203. The surface of this photosensitive drum 203 is evenly charged by a corotron 204 for primary electrostatic charging purposes. Then, the surface is exposed to a scanning laser beam 205 for image forming purposes so as to correspond to the image information, resulting in a latent image being formed on the surface. The latent image formed on the surface of each photosensitive drum 203 is developed by a developing unit 206 of each of the image forming units 200K, 200Y, 200M, and 200C, using toner of each color, i.e., black, yellow, magenta, and cyan, whereby visible toner images are formed. After having undergone electrification of a pre-transfer electrostatic charger 207, the visible toner images are transferred sequentially onto the transfer paper 201 retained on the transfer belt 202 by means of the electrostatic charge generated by transfer electrostatic chargers 208. The transfer paper 201 having the black, yellow, magenta, and cyan toner images transferred thereon is detached from the transfer belt 202. The thus-removed transfer paper 201 is subjected to fixing treatment by an unillustrated fixing unit, whereby a color image is formed.
On the drawing, reference numeral 209 designates a photosensitive material cleaner; 210, a photosensitive material charge-removing lamp; 211, a paper removal corotron; 212, a transfer belt charge-removing corotron; 213, a transfer belt cleaner; and 214, a pre-cleaning corotron.
The tandem color image forming apparatus having the above-described construction, is based on a scheme in which one image is formed by use of a plurality of image forming units, which enables considerably rapid formation of color images. However, an increase in the speed of formation of an image results in frequently occurring problems related to registration of the images formed by the image forming units; namely, problems related to registration of colors (hereinafter referred to as registration), which in turn makes it considerably difficult to achieve a high-quality color image and high-speed production rate. This problem is ascribed to slight variations in the position and size of components in each image forming unit as well as in the position and size of each image forming unit due to variations in the temperature within the color image forming apparatus and receipt of external forces. Of these factors, variations in the internal temperature of the image forming apparatus and the external force are impossible to prevent. For example, routine operations, such as correction of paper jams, replacement of components as a result of maintenance, or the movement of the color image forming apparatus, result in external forces exerted on the color image forming apparatus.
To prevent this problem, such an image forming apparatus as disclosed in, e.g., Unexamined Japanese Patent Publn. No. Hei-1-281468, has already been proposed. This image forming apparatus comprises a plurality of image forming sections that form visible images of position detection marks and visible images corresponding to original image information; a travel member that sequentially passes the transfer areas where the visible images, which are formed by the image forming sections so as to correspond to the original image information or the visible images of the position detection marks, are transferred; and position-detection mark detecting means that is disposed downstream of the transfer area in the direction of movement of the travel member and detects the position detection mark transferred on the travel member. The image forming apparatus is arranged to control the respective image forming sections in such a way as to correct the difference between transferred images on the basis of a detection signal output from the position-detection mark detecting means.
Where the technique of correcting the difference between transferred images is applied to the so-called tandem color image forming apparatus shown in FIG. 39, the four image forming units 200K, 200Y, 200M, and 200C, i.e., black, yellow, magenta, and cyan, form a plurality of color difference detection patterns, i.e., 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C, over the entire circumference of the transfer belt 202 at predetermined intervals in the direction of travel of the transfer belt 202 and the direction orthogonal to the direction of travel of the transfer belt 202, as shown in FIG. 40. A linear light-receiving element 222 such as a CCD sensor, which comprises a plurality of linearly-arrayed light-receiving pixels, samples the color difference detection patterns 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C using transmission light originating from a light-emitting element 223. Next, the intervals between the color difference detection patterns 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C are calculated. The positions of the image forming units 200K, 200Y, 200M, and 200C and image-formation timing are corrected such that the intervals become equal to a predetermined reference value, thereby realizing high-quality images. The color difference detection patterns 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C formed on the transfer belt 202 are removed by a transfer belt cleaner 213 after the sampling operation.
In the color image forming apparatus having the above-described configuration, the predetermined color difference patterns 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C are formed over the entire circumference of the transfer belt 202 by means of the image forming units 200K, 200Y, 200M, and 200C, as shown in FIG. 40. These color difference patterns 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C are detected by means of the linear light-receiving element 222 consisting of CCD sensors. The intervals between the color difference detection patterns 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C are calculated. The positions of the image forming units 200K, 200Y, 200M, and 200C and the image-formation timing are corrected such that the intervals become equal to a predetermined reference value, thereby realizing high-quality images.
However, the aforementioned color image forming apparatus has the following drawbacks: The color difference detection patterns 220K, 220Y, 220M, 220C, 221K, 221Y, 221M, and 221C are formed over the entire circumference of the transfer belt 202 that includes a seam 202a, as shown in FIG. 39. These color difference detection patterns 220 and 221 are removed by the transfer belt cleaner 213 after the sampling operation. The seam 202a of the transfer belt 202 has a minute step-wise gap. This gap makes it difficult to completely remove the color difference detections patterns 220 and 221 which are formed on the seam 202a of the transfer belt 202. As a result, the toner that forms the color difference patterns 220 and 221 remains on the seam 202a of the transfer belt 202. If the toner that forms the color difference patterns 220 and 221 remains on the seam 202a of the transfer belt 202 in this way, the remaining toner adheres to the reverse side of the transfer paper 201 held on and carried by the transfer belt 202 when the next color image is formed. Consequently, the reverse side of the transfer paper 201 is soiled with the remaining toner.
The previously-described color difference patterns 220K, 220Y, 220M, 220C, 122K, 122Y, 122M, and 122C are formed over the entire circumference of the transfer belt 202 that includes the seam 202a. As previously described, the seam 202a of the transfer belt 202 has a minute step, and hence the density of the color difference detection patterns 220 and 221 formed on the seam 202a of the transfer belt 202 may vary, or the color difference detection patterns may not be formed completely. If the density of the color difference patterns 220 and 221 formed on the seam 202a of the transfer belt 202 varies, or if the color difference patterns are not formed completely, a detection error will arise when the linear light-receiving element 222 detects the color difference patterns 220 and 221.
To prevent the above-described problem, it has been already proposed a sample-and-correction method in Unexamined Japanese Patent Publication 6-253151. According to this method, where the sampling and correction of an image are controlled, a sampling starting point and a sampling interval of sampling control means are set by means of control means. Repetitive patterns for measuring misregistration are generated. Sampled data and processed data are added up, whereby the positions of the patterns are determined. The accuracy of detection of the patterns for misregistration purposes is improved by setting the sampling starting point and the sampling interval of the sampling control means.
However, the above-described prior art has the following drawbacks. In the case of the image sample-and-correction method for use in registration of the multiple image forming apparatus as disclosed in Unexamined Japanese Patent Application No. Hei-6-253151, color misregistration having a constant magnitude and orientation (hereinafter referred to as DC color misregistration) arises because of minute variations in the position and size of components in the image forming unit as well as in the position and size of each image forming unit resulting from variations in the internal temperature of the color image forming apparatus and application of an external force to the image forming apparatus. This color misregistration is detected and corrected. The color misregistration also includes color misregistration whose magnitude and orientation periodically change (hereinafter referred to as AC color misregistration) in addition to the previously described DC components. The AC color misregistration is primarily caused by a rotating body such as a photosensitive drum and belt drive rollers. In the conventional color image forming apparatus, variations in the motion of the rotating body, such as a photosensitive drum and belt drive rollers, are detected by use of an encoder attached to the rotary shaft of the photosensitive drum, etc. The variations in the rotation of the photosensitive drum detected by the encoder, are fed forward or backwards to a drive motor so as to reduce the variations in the rotation of the photosensitive drum. Even if a control operation is carried out so as to reduce variations in the rotation of the photosensitive drum, there still exist the eccentricity of the photosensitive drum, the eccentricity of the surface of the photosensitive drum as a result of its attachment, and the eccentricity of the rotary shaft of the photosensitive drum or the belt drive rollers, due to clearance errors. These eccentricities cause AC color misregistration, which in turn results in degraded picture quality.
Further, in the conventional color image forming apparatus, the AC components arising during one cycle of the photosensitive drums 203 of the image forming units 200K, 200Y, 200M, and 200C, are out of phase with each other within a certain section of the transfer belt 202, as shown in FIG. 41, provided that one cycle of respective photosensitive drums 203K, 203Y, 203M, and 203C is T. This is also another cause of the degraded picture quality due to the AC color misregistration.